Support for transportable off-shore island

ABSTRACT

A LEG SUPPORTING A MARITIME PLATFORM COMPRISES A BUOYANT BASE HELD SUBMERGED BY A WEIGHT RESTING ON THE SEA BOTTOM, THE WEIGHT BEING TIED TO THE BASE BY A HOISTING CABLE WHOSE TENSION CONTROLS A PAIR OF VALVES FOR PARTLY FLOODING OR VENTING THE BASE TO REDUCE OR INCREASE ITS BUOYANCY IN A MANNER COMPENSATING FOR CHANGES IN THE LOAD OF THE PLATFORM.

March 2, 1971 H. RAMME SUPPORT FOR TRANSPORTABLE OFF-SHORE ISLAND FiledFeb. 24, 1969 Pressure Accumula for United States Patent U.S. Cl.61-46.5 3 Claims ABSTRACT OF THE DISCLOSURE A leg supporting a maritimeplatform comprises a buoyant base held submerged by a weight resting onthe sea bottom, the weight being tied to the base by a hoisting cableWhose tension controls a pair of valves for partly flooding or ventingthe base to reduce or increase its buoyancy in a manner compensating forchanges in the load of the platform.

My present invention relates to a transportable offshore island whereina load-carrying structure, such as a steel platform, is mounted onseveral (usually three) legs rising from the bottom of the sea or someother body of water in which the island is to be stationed, e.g., forwelldrilling purposes.

The stationing of such structures on legs directly resting on the seabottom entails the risk that the legs would gradually sink into therelatively soft ground. It has therefore already been proposed to mountthese legs on a buoyant base held submerged by weighted anchors Whichare tied to the structure by cables sloping in different directions andare equipped with grappling hooks or the like to resist peripheral andcentripetal forces resulting from the torque of the drilling operationand the buoyancy of the base. The Weight of these anchors must beconsiderable in order to ensure that the island will not be cast adriftupon a major reduction in load or in response to severe buffeting bywinds and waves.

The placement and relocation of these anchors, accordingly, is adifficult and time-consuming maneuver.

The general object of my present invention is to provide a support for amaritime platform, or other structure rising from a body of water, inwhich the aforedescribed difliculties are avoided and which thereforecan be anchored and refloated with a minimum of effort.

Another object is to provide means in such system for maintaining thebuoyancy of a submerged support within a predetermined, relativelynarrow range independent of loading conditions, thus enabling thesupport to be held down by a relatively small weight.

A related object of my invention is to provide support means for such astructure whose lifting force under water is substantially constant andlow enough to be overcome by a weight or weights so small as to becarried by the support means upon a floating of the latter.

I realize the aforestated objects by the provision of sensing meansresponsive to the tension of a cable or other tie means linking thesubmerged base with its anchoring weight or weights, this tension beinga measure of the buoyancy of the base as modified by the load carried onthe supported structure, in combination with control means such as a setof valves for varying this buoyancy in a manner compensating for changesin load.

In a preferred embodiment, the base includes a framework rigid with aplurality of generally pontoon-shaped floats, three such floats beingnormally suflicient to carry a platform equipped with a well-drillingrig. Each float has a bottom opening or well through which a cable froma hoist anchored therein extends substantially vertiice cally to engagea weight which, during the stationing of the platform, is loweredthrough that opening onto the ocean floor; a subsequent tensioning ofthat cable by the hoist drive, after a reduction of the buoyancy of thefloat by additional loading and/or by partial flooding, then draws thebase under water oncoming to rest on the ocean floor, the float willreceive the weight in its bottom well. Upon submersion the tensionsensor associated with the hoist is made operative to maintain anapproximately constant buoyancy in the submerged floatv With the centerof gravity of the float substantially coinciding with a vertical axispassing midway through the bottom opening thereof, and with aplatform-supporting leg rising from the float substantially in line withthat axis, the position of the submerged base will generally be stableagainst both lateral and rotary stresses. Naturally, conventionalgrappling hooks of negligible weight could be used in addition, ifnecessary, to secure the structure against horizontal motion in anydirection.

An advantageous arrangement for sensing the tension of the hoistingcable, and therefore the magnitude of the lifting force exerted by thefloat, involves the anchoring of the hoist to a mobile member,preferably a generally horizontal lever arm, the buoyancy-control meansincluding a pressure gauge coupled directly or indirectly to that arm.The restoring force needed to counteract the tensioning of the hoistingcable may be supplied at least in part by the reaction of this pressuregauge, supple mented if necessary by springs and/or counterweights.

The above and other features of my invention will become more clearlyapparent from the following detailed description of a preferredembodiment illustrated, partly diagrammatically and with portions brokenaway, in an elevational sectional view constituting the sole figure ofthe accompanying drawing.

The system shown in the drawing comprises a platform 20 supported by abase 21 which includes several structurally identical floats 1, 1' (onlytwo shown), a framework 19 rigidly interconnecting these floats, and aset of legs 2 (only one shown) rising from the center of each float tothe underside of the platform, e.g., at a respective corner of itstriangular outline. The leg 2 is a tubular column which, in a mannerwell known per se, may permit a crew member to descend from the platform20 to the interior of the float by way of an air lock represented by apair of hatches 3a, 3b.

The axis A of column 2 passes through the center of gravity G of float 1and also through the center of a bottom opening or well 22 which may beof circular, square or other polygonal configuration. A heavy block 10,fitting with some clearance inside the well 22, is connected with thefloat 1 by a hoist 11 including an upper roller 11a, a lower roller 11band a deflecting roller 11c. Roller 11a is mounted by a strap 12 on alever 13 having a fulcrum 15 on a post 23; the cable 11d of the hoisthas its fixed end secured to the lever 13 at 24 and has its opposite endwound about a reel 14 driven by a reversible motor 25. The reel 14 andthe motor 25 are both shown supported on the lever 13 in the vicinity ofits fulcrum while the opposite, free end 16 of the lever is connectedwith a conventional pressure gauge diagrammatically illustrated at 17.This pressure gauge, apart from resisting a downward swing of the leverend 16, is hydrau lically, pneumatically or mechanically coupled with arotary switch 18 controlling a pair of normally closed solenoid valves 4and 8 by way of respective bank contacts 18a and 18b. A further bankcontact leads to a control circuit 26 for drive motor 25. The operatingcircuits of valves 4 and 8 can be disconnected, as shown, by a circuitbreaker 27 remote-actuated from a panel on platform 20 of relay circuitsnot further illustrated. Valve 4, when opened, allows air from theinterior of float 1 to escape into the ambient air or water, dependingon whether the pontoon 1 is floated (as shown in full lines) orsubmerged (with a limiting position shown in dot-dash lines), whereasvalve 8 in series with a check valve 9 allows in its open state theintroduction of air under pressure into the pontoon through a conduit 7from a pressure accumulator 8 which is fed by a compressor 5 on platform20.

Let us assume, by way of example, that the anchor block 10 has aneffective weight of 500 tons and that the individual buoyancy of eachfloat 1, 1', etc., when submerged, is to be maintained at approximately300 tons. During transportation of the structure to the anchoring sitethis block is retracted into the well 22 of the float 1 which,naturally, must then be loaded lightly enough and contain a large enoughvolume of air to float with the weight 10 suspended therefrom; thus, theuplift of the pontoon loaded down by a proportional part of thesuperstructure 2, 19, 20 must be greater than 500 tons under theconditions assumed. Upon arrival at the site, motor control 26 is set inmotion by a signal from the operators cabin aboard the platform 20 todrive the reel 14 in a sense slowly paying out the cable 11d until theweight 10 hits bottom; during this operation, with the circuit breaker27 open to prevent any untimely opening of valves 4 and 8, gauge 17measures the effective weight of block 10 and holds the switch 18 offits bank contact 180. When the block 10 touches ground, the tension ofhoisting cable 11d decreases while the float 1 begins to rise higherabove the water level with continuing rotation of reel 14. When thetension measured by gauge 17 has dropped to a certain level, e.g. of 200tons, switch 18 engages its contact 180 to stop the motor 25.

After the weights of all the pontoons interconnected by framework 19have been thu stabilized, the platform 20 is fully loaded with remotemanipulation of valve 4 or 8 from the operators cabin (if necessary) toreduce the eflective buoyancy of each float from its previous value inexces of 500 tons to a value lower than 500 tons so that subsequentoperation of the hoist 11 will submerge the float instead of lifting theweight 10. The operator on platform 20 then actuates the motor control26 'to drive the reel 14 of each float in the reverse sense, therebytightening the cable 11d of the corresponding hoist and pulling the base21 under water. Prior to that time or just before the base alights onthe ground, circuit breaker 27 is closed so that switch 18 is able tocontrol the buoyancy of the float 1 by opening the air-inlet valve 8 viacontact 18b when the cable tension is less than a predetermined lowerlimit, e.g., 290 tons, and by opening the air-outlet valve 4 when thattension exceeds a predetermined upper limit, e.g., 310 tons, so as tomaintain the buoyance per float at approximately 300 tons. If the loadon the platform 20 is increased by up to 300 tons per supportingpontoon, the residual buoyance will prevent any settling of the base 21in the ocean floor; larger loads will have to be placed thereon moregradually, at a rate determined by the response time of the controlsystem, up to an upper limit dependent on the maximum buoyancy of thefloats fully cleared of water.

The switch 18 may be provided with additional contacts, not shown, forkeeping the valve 4 or 8 open until the cable tension as sensed by thegauge 17 reaches a value in the vincinity of the desired 300-ton levelafter rising above or falling below the assumed threshold of 310 or 29 0tons, in a manner analogous to the operation of conventional thermostatsor similar range-setting controls. In principle the buoyancy levelmaintained by this regulator could be a much smaller fraction of thetotal load, particularly if major load increases during operation arenot to be expected.

The coupling of the pressure gauge 17 with hoist 11 through theintermediary of oscillatory member 13 renders the reading of that gaugeindependent of any frictional resistance encountered by the cable 11d inits passage around rollers 11a, 11b and 11c and reel 14.

Naturally, the various elements and other sensitive devices within float1 should be protected against the intruding sea water by suitableshielding not shown.

I claim:

1. A support for a structure rising from a body of water, comprising afloat with a bottom opening; lever means in said float mounted abovesaid opening with limited swingability about a horizontal fulcrum;weight means resting on the ground in said body of water; hoist meansanchored to said lever means and said weight means while traversing saidbottom opening; reversible drive means coupled to said hoist means forlowering said weight means to the ground and for subsequently drawingsaid float under the water level; first valve means near the top of saidfloat for flooding said float by allowing the escape of air from itsinterior; second valve means near the top of said float for expellingwater from the interior thereof by the admission ofa gas under pressure;and sensing means coupled to said level means for resisting a swing ofsaid lever means under stress from said weight means and for selectivelyoperating said first and second valve means to vary the buoyancy of saidfloat in a manner compensating for changes in the loading of saidstructure.

2. A support as defined in claim 1 wherein said opening is formed by awell for receiving said weight means in the body of said float.

3. A support as defined in claim 1 wherein said lever means comprises asubstantially horizontal arm overhanging said opening, said hoist meansincluding a cable with a fixed end secured to said arm and a reel onsaid lever means engaging a mobile end of said cable, said drive meansbeing operatively connected with said reel.

References Cited UNITED STATES PATENTS 2,972,973 2/1961 Thearle 6l46.5X3,031,997 5/1962 Nesbitt 6l46.5X

FOREIGN PATENTS 1,012,370 12/1965 Great Britain 61-465 JACOB SHAPIRO,Primary Examiner US. Cl. X.R. 114-.5, 43.5

